The invention relates to a picture display device having a display tube whose rear portion has of a cylindrical neck accommodating a device for generating electron beams and whose front portion is funnel-shaped, the widest portion being present on the front side and having a phosphor display screen. The display device also has an electro-magnetic deflection unit mounted around a part of the display tube for deflecting electron beams across the display screen, the unit including a line deflection coil having two line deflection coil halves arranged on either side of a plane of symmetry and a field deflection coil and a compensation coil system for generating a magnetic compensation field which is oppositely directed to the line frequency radiation field in a space in front of the display screen.
EP-A 220,777 discloses such a display device having means for compensating stray fields produced by the line deflection coil.
Recently more stringent standards have been introduced for certain types of picture display devices, notably for monitors, with respect to the magnetic interference field which they may produce around them. An important source of magnetic interference fields is the line deflection coil because it is operated at radio frequency currents (frequencies in the range of 10 to 100 kHz) as contrasted to the field deflection coil. It is impossible to design a satisfactorily operating deflection coil which does not produce a stray field. If the stray field were to be eliminated by means of a protective shield, such a shield would only be effective if the combination of display tube and deflection unit were also shielded on the display screen side. It is true that the external magnetic field of a deflection unit is not very strong; at a distance of 50 cm from the front side of a deflection unit for a 110.degree. monochrome display tube the field strength has already decreased to approximately 1% of the strength of the earth's magnetic field, but it is the variation of the field with respect to time which is important. Field variations may cause interferences in other electronic apparatus, and research is being done to establish whether human health is affected by these fields. Nowadays the time derivative of the field of the deflection unit increases with the increase of the line frequencies and hence with increasingly shorter fly back periods.
For compensating the line deflection stray field the use of a compensation coil system which, when energized, generates a compensating magnetic dipole field is described in the above-mentioned Patent publication. This dipole field can be obtained by energizing one coil whose turns are mainly located in one flat plane (a "current loop"), which coil has the correct number of turns, the correct surface area and the correct orientation. Energization may be effected, for example, by arranging the compensation coil in series with or parallel to the line deflection coil. The compensation field may be obtained alternatively by energizing two "current loops" which are positioned on either side of the line deflection coil, which current loops have the correct number of turns, the correct surface area and the correct orientation. Also in this case energization may be effected, for example, by arranging the compensation coils constituted by the current loops in series with or parallel to the line deflection coil.
The compensation coils are preferably large so as to reduce their energy content.
However, a problem is that many types of display devices (particularly monitors) lack the space to accommodate large coil systems in their correct position. Consequently, relatively small (too small) compensation coils must be used so that the radiation compensation consumes much (line deflection) energy. Moreover, the sensitivity of the line deflection coil is detrimentally influenced if the compensation coil system is arranged in series with the line deflection coil. The induction then increases.